This application is a national stage filing under 35 U.S.C. 371 of International Application number PCT/IE99/00062, filed Jul. 8, 1999, and published by the International Bureau, in English, Jan. 20, 2000.
This invention relates to an improved process for preparing N-phosphonomethyl iminodiacetic acid (PMIDA) also known as N,N-diacetic acid aminomethylene-phosphonic acid. PMIDA is an intermediate in the preparation of N-phosphonomethyl glycine (PMG) also known as glyphosate, which is a well-known translocated, postemergence, broad-spectrum herbicide.
A process which is currently widely used for preparing PMIDA involves reacting an alkali metal salt of iminodiacetic acid (IDA) in aqueous strong mineral acid solution with phosphorous acid and formaldehyde. Such a process is described in U.S. Pat. Nos. 4,724,103 and 4,775,498. In this process the strong mineral acid salt of IDA and the alkali metal salt of the strong mineral acid are formed initially. The strong mineral acid salt of IDA has a low solubility, so that the alkali metal salt of the mineral acid cannot be removed easily at this stage and therefore remains in the reaction medium. Subsequent phosphonomethylation of the IDA mineral acid salt results in a mixture of PMIDA and the alkali metal salt. The latter salt is separated from the PMIDA by the addition of water to the mixture in a quantity sufficient to dissolve the alkali metal salt. The PMIDA is then recovered from the solution by precipitation and filtration. The main disadvantage of this process is that large quantities of effluent are generated because of the necessity to add water to dissolve the alkali metal salt, which complicates the feasibility of recycling this liquor which contains valuable raw materials.
It is therefore an object of the present invention to provide a process for preparing PMIDA which avoids or minimises the disadvantages of the prior art.
According to the present invention there is provided a process for preparing N-phosphonomethyl iminodiacetic acid (PMIDA), which process comprises the following steps:
(a) reacting an alkali metal salt of iminodiacetic acid (IDA) with phosphorous acid and a strong mineral acid or a source of these acids to form iminodiacetic phosphite and an alkali metal salt of the strong mineral acid;
(b) removing at least a part of he alkali metal salt formed in step (a);
(c) reacting the iminodiacetic phosphite formed in step (a) with phosphorous acid and a strong mineral acid or a source of these acids, together with formaldehyde, to form PMIDA in reaction medium; and, if desired,
(d) isolating the PMIDA; and
(e) optionally recycling the medium remaining following the isolation of the PMIDA in the process.
The alkali metal salt of IDA is preferably the disodium or dipotassium salt.
Strong mineral acids which may be used in the process of the invention include hydrochloric, hydrobromic, hydriodic and sulphuric acids. Hydrochloric acid is preferred.
A particularly preferred source of phosphorous acid is PCl3 which can also provide hydrochloric acid when hydrolysed in aqueous medium.
The intermediate compound, iminodiacetic phosphite (IDA.H3PO3), is novel and forms a further aspect of this invention.
In step (a) of the process of the invention, the amount of strong mineral acid added or generated should be just sufficient to neutralise the IDA alkali metal salt and any free alkali (such as NaOH) present, preferably about 0.5 to 1.5, more preferably about 0.5 to 1.0, equivalents of acid. Excessive amounts of the strong mineral acid must be avoided as any excess will lead to the formation of the mineral acid salt of IDA which is not desired in the present process. In the present process, the reaction of the IDA salt with phosphorous acid and a strong mineral acid or a source of these acids, such as PCl3, leads to the formation of the novel intermediate, iminodiacetic phosphite, and the alkali metal salt of the strong mineral acid. After cooling to about 85xc2x0 C., the alkali metal salt is removed, conveniently by filtration, in an amount sufficient to avoid coprecipitation of the salt with the final product (PMIDA). Because iminodiacetic phosphite is highly soluble in the reaction medium and the alkali metal salt is not, the alkali metal salt can be readily removed at this stage (step (b)).
The reaction in step (a) is carried out at a temperature in the range of from 40xc2x0 C. to 75xc2x0 C., preferably about 45xc2x0 C. to 65xc2x0 C. for about 1-4 hours. The reaction mixture is cooled to a temperature between 80xc2x0 C. and 90xc2x0 C. before removal of the alkali metal salt of the mineral acid. The iminodiacetic phosphite solution at this stage is maintained at a temperature greater than 60xc2x0 C., preferably between 70xc2x0 C. and 100xc2x0 C.
In step (c), the iminodiacetic phosphite is reacted with a further amount of phosphorous acid and a strong mineral acid or a source of these acids, and formaldehyde at a temperature in the range of from 105xc2x0 C. to 135xc2x0 C. for a period of about 30 minutes to 4 hours. After cooling to about 30xc2x0 C., PMIDA is formed as a solid precipitate in the reaction medium. Subsequent separation of PMIDA (step (d)) can be achieved by conventional methods such as filtration or centrifugation.
The reaction medium from step (d) is preferably recycled in the process of the invention. In the recycling process, alkali such as NaOH is added to the iminodiacetic phosphite if necessary to at least partially neutralize free acid such as HCl. Phosphorous acid and a strong mineral acid, or a source of these acids such as PCl3, are added to the recycle medium. Following reaction at a temperature between 45xc2x0 C. and 65xc2x0 C. for a period of about 30 minutes to 4 hours, the reaction product is cooled to about 20xc2x0 C.-40xc2x0 C. and a sufficient amount of the alkali metal salt of the strong mineral acid is removed as a wet cake, conveniently by filtration, to allow recycling. The salt is easily removed at this stage because the presence of free acid is high. Where the salt is NaCl and the acid is HCl, it is well known that the solubility of NaCl in water in the presence of 28-30% HCl is essentially zero. A portion of the resultant filtrate containing H3PO3 and strong mineral acid is then reacted with the alkali metal salt of IDA to form iminodiacetic phosphite and the alkali metal salt of the strong mineral acid, as described hereinabove for step (a). The amount of strong mineral acid present should not be excessive but should be sufficient to neutralize any free alkali and IDA salt, so that iminodiacetic phosphite and not the strong mineral acid salt of IDA is formed. The alkali metal salt is then removed as described above.
The resulting iminodiacetic phosphite solution is kept hot, typically at a temperature greater than 60xc2x0 C. and preferably between 70xc2x0 C. and 100xc2x0 C. Water can be boiled off at this stage if necessary to ensure proper water balance and maximum liquor recycle The remainder of the filtrate generated above is then added to the iminodiacetic phosphite solution, together with formaldehyde and reacted as described above for step (c) to form PMIDA and further reaction medium which can be recycled in the process.
Iminodiacetic acid (IDA) or a salt thereof can be prepared as described in U.S. Pat. Nos. 5,689,000; 5,620,584; or 4,782,183; or 3,904,668 or WO-A-94/24091. IDA can also be obtained commercially and the desired salt prepared in known manner.
The yield of PMIDA prepared according to the process of the invention can be as high as 91%. Furthermore, because the alkali metal salt of the mineral acid formed during the reaction can be removed as a wet cake, the mother liquor containing valuable raw materials can be recycled thereby greatly reducing the amount of effluent generated and increasing the efficiency of the process in an environmentally friendly manner. The mother liquor can contain approximately 5% free HCl based on a 60% liquor recycle. Higher recycle rates, such as 80%, can also be achieved as can be seen in the Examples hereinafter.
Unless otherwise stated, the disodium iminodiacetate (DSIDA) used in the Examples was prepared by neutralizing iminodiacetic acid (supplied by Aldrich) with 2.2 mole equivalents of 50% NaOH.